Comps Index

The Crandall Conglomerate crops out in the Clarks Fork of the Yellowstone River Valley in northwestern Wyoming. The conglomerate was deposited in an ancient valley system incised at least 160 m into Paleozoic strata. The Heart Mountain Fault cuts the Crandall Conglomerate, cleanly slicing some clasts, indicating that the conglomerate was lithified before faulting. Conglomerate exposures in the footwall follow a structural anomaly in the Cambrian Pilgrim Limestone called the Blacktail Fold. This suggests a structural control on the position of the paleovalleys.

The Crandall is a coarse deposit, the largest clasts averaging 15 cm in diameter. About 95% of the clasts are carbonate and were derived from the Paleozoic limestone units through which the valleys were cut. A small fraction of the conglomerate is assorted chert and granite clasts from Precambrian basement granite exposed at the head of the valley system. The dominant facies consists of unsorted, ungraded, clast-supported conglomerate with well rounded cobble-sized clasts. Less common facies include matrix-supported, sub-rounded conglomerate, pebble and granule conglomerates, very coarse grained sandstone, and fine grained red sandstone. All facies occur in discontinuous layers with either gradational or sharp bounding surfaces.

The formation is unusual because of a conspicuous lack of fine-grained matrix in the conglomerates and the sparsity of fine-grained facies overall; no fossils or organic matter have been found in the conglomerate. A depositional model is proposed in which deposition of coarse conglomerate by catastrophic flow was followed by reworking of these sediments and addition of new sediment by normal stream processes.

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